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CCC does not believe that more accurate modeling results from simply increasing <br />the Curve Numbers without any technical or site specific basis, although doing so <br />clearly produces more conservative modeling (i.e., larger runoff). The root of the <br />issue is the selection of Hydrologic Soil Group (HSG) for the reclaimed lands. This <br />affects the selection of Curve Number in the various USDA/NRCS published tables <br />that relate curve numbers to specific land use and surface condition for a specified <br />HSG. Selection of the runoff curve number (CN) in turn directly affects the <br />magnitude ofrunofffor any modeled storm event. <br />Colowyo Coal Company identified the soils on the mine site as HSG C only as a <br />condition of approval of TR -73. Colowyo Coal Company's professional <br />engineering hydrology consultants do not believe that reclaimed soils should be <br />assigned a HSG C in this case. In the review process of TR -73, in the May 11, <br />2009 response to the agency's second round comments, CCC explained the basis of <br />the assignment of HSG B to reclaimed areas, elaborating on the concept that <br />wholesale removal and replacement of existing soils destroys all shallow soil <br />structure, and would be expected to result in a higher infiltration rate than the <br />native undisturbed soil. This approach is consistent with the recommendations <br />for `Disturbed soils" presented on page 7 -5 of Part 630 of the NEH, and is also <br />strongly supported by the results of the OSM sponsored 1981 Striffler and Rhodes <br />study. The technical arguments presented in the TR -73 review process and the <br />results of the 1981 infiltration study were also the basis for the selection of HSG <br />T" soil for reclaimed lands in Exhibit 7 Hydrology Methodology and Assumptions <br />as submitted with TR -56 in 2003. The previous hydrologic consultants (Western <br />Water Consultants) and the current consultant (Environmental Solutions, Inc.) <br />believe the approach is sound and is in accordance with accepted engineering <br />practice as indicated by their engineering certifications on the several submittals. <br />In the end however, CCC was unable to persuade CDRMS staff that this approach <br />is correct and agreed to delete from Table 1 the column that had historically <br />addressed the Hydrologic Soil Group (HSG), in particular showing the selection of <br />HSG B for reclaimed soils. CCC also added a note in the footnotes of Table 1 to <br />reinforce that all soils would be considered as HSG C The sentence on the page <br />previous to Table 1 which stated that reclaimed soils should be treated as HSG B <br />was not removed. That sentence, and the overall approach, had been in place <br />since at least the TR -56 submittal. CCC suspects that it is also the basis of the <br />hydrologic modeling of other nearby mines which also assign HSG B to reclaimed <br />soils and use curve numbers in the 60s for reclaimed lands. <br />This TR -95 seeks only to move Curve Numbers for reclaimed lands part of the <br />way back to their historic values. Those were consistent with a strict reading of <br />